Study of (AT/AZ) as compared to (TMP/SMX) in the prevention of serious bacterial infections in children with HIV infection. The primary objectives of the study are: to determine the comparability of each arm in the prevention of serious bacterial infections, to determine the long term safety and tolerance of these drugs, and to determine if there is pharmacokinetic interaction between Atov-Aquone and Azithromycin. (ACTG 254 V. 3)